Prevent not-* from being used with variants with multiple sibling rules

[thecrypticace]

Variants like this can't be easily negated by our current system:

@custom-variant dark {
  &.is-dark {
    @slot;
  }
  @media (prefers-color-scheme: dark) {
    @slot;
  }
}

Right now it produces the following CSS which is logically incorrect:

.utility-name {
  &:not(.is-dark) {
    /* ... */
  }

  @media not (prefers-color-scheme: dark) {
    /* ... */
  }
}

The correct CSS is this which requires moving things around:

.utility-name {
  @media not (prefers-color-scheme: dark) {
    &:not(.is-dark) {
      /* ... */
    }
  }
}

We're opting to disable this instead of generating incorrect CSS for now. I'd like to bring this back in the future for simpler cases in the future.

[thecrypticace]

@RobinMalfait we should probably merge your custom-variant PR first.